|  | 5.1 | The Network Layer Basics
| 5.1.3 | The operation of routing tables
In IP networks, the router forwards packets from the source network to the destination network based on the IP routing table. After the router determines which path to use, it can proceed with switching the packet. This means it accepts the packet on one interface and forwards it to the next hop interface on the best path to the packet's destination. This is why routing protocols are so important, because each router that handles the packet must know what to do with the packet. Routing tables store information on possible destinations and how to reach each of the destinations. Routing tables need to store only the network portion of IP addresses for routing. This keeps the tables small and efficient. Entries in the routing tables contain an IP address of the next hop along the route to the destination. Each entry specifies only one hop and points to a router that is directly connected, which means that it can be reached across a single network. Routing protocols fill routing tables with a variety of information. For example, a router uses the destination/next-hop routing table when it receives an incoming packet. The router uses its routing table to check the destination address and attempts to associate the address with a next hop. A destination/ next-hop routing table tells a router that a particular destination can be best reached by sending the packet to a particular router that represents the next hop on the way to the final destination. Routers must communicate with each other in order to build tables through the use of routing protocols and through the transmission of a variety of messages. The routing update message is one such message. Routing updates generally consist of all or a portion of a routing table. By analyzing routing updates from all routers, a router can build a detailed picture of network topology. When routers understand the network topology, they can determine the best routes to network destinations.
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